High temperature polymer filtration medium

ABSTRACT

A filter medium for use in particulate filtering applications is disclosed. The filter medium is formed from a substrate capable of retaining the desired physical structure of the filter medium under the conditions of the filtering application, including temperature. A polyimide based stiffening agent adapted for treating the substrate is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 10/000,236, which was filed on Nov. 30, 2001, andissued as U.S. Pat. No. 6,752,847 on Jun. 22, 2004.

BACKGROUND OF INVENTION

A technique for controlling pollutants and emissions from industrialplants is to remove undesirable particulate matter carried in a gasstream by fabric filtration. Such fabric filtration is accomplished in aparticulate matter or dust collection apparatus known in the industry asa “baghouse.” The fabric filter is preferably pleated to increase theeffective filtering area while occupying the same or less space. Theimproved baghouse illustrated herein includes two large chambers, orplenums, that are divided by a tube sheet having a plurality of openingsfor receiving air filter cartridges.

The filter cartridges include a pleated filter medium that retains theunwanted particles as the air is forced through the cartridge.Traditional dust filtration materials are made from woven or needlepunchmedia. Newer filters, including higher surface area pleated media, aremade from spunbond or other nonwoven media. The effectiveness of thefilters diminishes as particles collect on the outer surfaces of thefilters, thereby diminishing air flow. To remove accumulated particleson the filters without physically removing the filters from thebaghouse, air may be pulsed through the baghouse cartridge filters fromthe opposite direction of air flow during particulate removal. Thereverse pulses abruptly and temporarily expand the filter media todislodge the particles, which fall to the bottom of the baghouse and areremoved. The effectiveness of the baghouse is greatly increased withouthaving to repeatedly remove and replace the cartridges.

Unfortunately, there are limitations associated with the use of pleatedfilter media. First, the pleated structure is sensitive to temperature.Complications arise when these media are being processed andmanufactured at temperatures similar to the temperatures used in thebaghouse during filtering applications. The higher temperature causesthe media to soften, allowing a level of pleat collapse or pleat pinchto occur. Pleat collapse can restrict air flow and cause increasedpressure drop minimizing the advantages of the higher filter surfacearea of the pleats. For this reason, the use of polymer filter media foruse at higher temperatures has been limited. (For purposes of thisdisclosure, high temperature is meant to include but is not limited tofiltering applications ranging from about 300° to about 500° F, withsurge temperatures typically reaching about 550° F.).

There are several primary fibers or polymers, processed into fabrics andused in the 300° to 550° F. range. Traditionally they are polyphenylenesulphide (PPS, with trade names such as Ryton®, Torcon® and Procon®) andaramid (Nomex®, Conex®) both of which operate up to but preferably below400° F. Applications operating at temperatures greater than 400° F. andup to 550° F. typically utilize fibers of glass, polyimide (P84) or PTFE(Teflon®, Profilen®, Rastex®). For this reason the selection of thesubstrate fiber or resin is in part determined by the conditions underwhich the filter will be used.

Second, the abrupt expansions from reverse pulse cleaning procedureplaces additional stress on the filter media. The stress results incracking of the resin or stiffening agent, discussed below, therebyshortening the effective filter life. As the tensile and tear strengthof the stiffening agent are lessened, the media tend to crack, splitand/or tear, leading to the escape of unfiltered emissions through thetears. Thus, the combination of stresses due to the inward filteringflow and outward cleaning pulses reduce the effective filter lifetypically to six months or less when operated twenty four hours/sevendays a week at a temperature of 350° F. to 400° F.

Conventional polymer filtering materials, including PPS based filteringmaterial, has heretofore been treated with stiffening agent resinsystems applied in a secondary process. The stiffening resins impregnatethe substrate to strengthen and stabilize the filter, as well as aidingin pleating and pleat retention. These known stiffening resins includeemulsions and/or dispersions of bisphenol based epoxies, acrylic basedresins, melamine and phenol formaldehyde resins. These resins arecommonly used in the textile industry and are recommended for use withtextile products exposed to high temperatures. They are applied toimpart the necessary features to allow the media to pleat and retain itsform and shape at the desired application temperatures. The resins aretypically applied via an impregnation process, where the media istotally immersed in a bath of the specified resin solution and then nipsqueezed to remove the excess solution prior to drying. After drying themedia is rendered stiff. The media is measured to ensure the correctlevel of resin has been applied, typically 10% to 25% add on to thebasis weight of the base or substrate medium.

These conventional resins are not fully cross-linked or cured wheninitially dried onto the fabric. This allows the media to re-softenduring subsequent high temperature processes, specifically the pleatingprocess, where the softening allows the fabric to conform to thepleating action. After subsequent cooling the resin helps maintain thepleat structure. Unfortunately, upon initial exposure to elevatedtemperatures during filtering applications the media softens, and doesnot fully cure and re-stiffen for up to several hours. Once fully cured,the fabric is better able to withstand the rigors of reverse airpulsing, at low temperatures, without losing shape and form. However,pleat collapse or pinching can occur while the material is soft prior tocuring. These resins that cure in two stages are called “B” stagedresins. The initial drying and partial curing is commonly known as“B-curing”, followed by a complete or final curing.

While these stiffening agent resins render the treated filter fabricpleatable, conventional resins cannot withstand the mechanical wear andrigors of reverse air pulse cleaning, as these resins tend to crack.Commercially available filter media typically have a life expectancy,dependent on conditions, of about six months, due to the thermal andmechanical wear on the stiffening resin.

It is therefore desirable to provide a polymer stiffening agent capableof withstanding the temperature and cleaning pulses of high temperatureindustrial baghouses, and to provide a pleated filter medium utilizingthe improved polymer stiffening agent.

SUMMARY OF INVENTION

The present invention relates to a high temperature polymer resin foruse as a stiffening agent for a filtering medium. More specifically, apolyimide treated pleated filter medium for use in a high temperatureindustrial particulate filter apparatus is disclosed.

The present invention is directed to an improved polymer stiffeningresin for use with a filtering medium. The stiffening resins of thepresent invention are polyimide (PI) based resins. Polyimides are ageneric class of condensation polymers containing an ‘imide’ linkage asa linear or heterocyclic unit along the polymer backbone.

In a preferred embodiment the polyimide stiffening resins are applied towoven and non-woven polymer substrates to form pleated filter media. Thenon-woven substrates are calendered, and both types of substrates areimmersion coated with the polyimide stiffening resin. The improvedfilter media of the present invention are especially well suited for usein applications at temperatures exceeding 300° F, such as industrialbaghouse filtering applications.

The present invention is also directed to a method of making the filtermedium of the present invention. A preferred method comprisescalendering a substrate; treating the calendered substrate with apolyimide stiffening agent; and curing the treated substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view, partly in section, of a baghouse shown withfilter assemblies containing the improved pleated filter medium of thepresent invention installed.

FIG. 2 is a perspective, enlarged sectional view of the filter assemblyillustrated in FIG. 1.

FIG. 3 is an elevational view of the filter assembly of FIG. 2.

FIG. 4 is a cross-sectional view of the filter assembly of FIG. 3 takenalong line 4-4 of FIG. 3.

FIG. 5 is an SEM (scanning electron microscope) photograph of anon-woven aramid polymer substrate treated with the preferred polyimidestiffening resin of the present invention.

FIG. 6 is an SEM photograph of a non-woven PPS/polyimide polymersubstrate treated with the preferred polyimide stiffening resin of thepresent invention.

FIG. 7 is a graph entitled “Combined Average MD and XMD Retained TensileStrength of PAI Resin Treated Aramid versus Epoxy Resin Treated Aramid.”

FIG. 8 is a graph entitled “Combined Average MD and XMD Retained TearStrength of PAI Resin Treated Aramid versus Epoxy Resin Treated Aramid.”

DETAILED DESCRIPTION

An improved polymer stiffening agent adapted for using the pleatedfilter media of the present invention is shown in FIG. 1, and isdisclosed in U.S. Pat. No. 6,203,591 B1 to J. T. Clements et al., whichis incorporated herein by reference. This baghouse is for purposes ofillustration only, and is not meant to limit the use of the presentinvention to this particular baghouse. The baghouse, generallydesignated by numeral 10, is defined by an enclosed housing 12 that isdivided into a “dirty air” plenum 14 and a “clean air” plenum 16 by atubesheet 18. A dirty air inlet port 20 is in fluid communication withthe dirty air plenum 14 and a clean air outlet port 22 is in fluidcommunication with clean air plenum 16. The tubesheet 18 includes aplurality of openings 24 sized to accept and retain a plurality offilter assemblies generally designated 26, as shown.

A filter cartridge 26 adapted for use with the present invention isillustrated in FIGS. 2-4. The filter cartridge 26 includes a fabricfilter 28 that is generally tubular and includes a number of pleats 30.The pleats have in inner surface 32 and an outer surface 34. In apreferred embodiment, pleats 30 abut an inner screen 36 defining acentral passageway 38 formed within filter cartridge 26. The fabricfilter 28 and screen 36 are aligned and held in place by an upper cap 40and a lower cap 42. Upper cap 40 includes a projecting shoulder 44 thatholds the cartridge in place against tubesheet 18 and prevents thecartridges from falling through openings 24. The assembly 26 is furtherstrengthened by a fabric strap 46, described in more detail in U.S. Pat.No. 6,233,790 B1 incorporated herein by reference.

In operation, the dirty air enters dirty air plenum 14 through inletport 20 and is filtered through the filter assemblies 26. As the airmoves through the filter media 28 into the central passageway 38particles are trapped against outer surface 34. The cleaned air thenexits passageway 38 into the clean air plenum 16 and is removed frombaghouse 10 through outlet port 22, as shown by the arrows in FIG. 1. Asthe filtered particles build up against the outer surface 34 of filter28, the efficiency of baghouse 10 decreases. In order to remove theparticles, air is pulsed in the reverse direction of airflow during thefiltering application. Air is pulsed at a desired rate to maintain thedesired airflow during filtering application. The reverse pulse “blows”the particles off the outer surface 34 of filter 28. The particles fallto the bottom of dirty air plenum 14 and can be removed.

While the illustrated baghouse, cartridge and strap are preferred, it isunderstood that any suitable baghouse, cartridge and/or strap design maybe utilized with the present invention. Further, the filter mediumdescribed below can be used in other filtering configurations.

The filter media of the present invention is comprised of a substrate,for example a polymer needlefelt medium, that has been impregnated witha polyimide resin stiffening agent. The polyimide stiffening resinprovides increased fiber strength and stabilization for pleat retentionand structural integrity during filtering and pulse cleaning, especiallywhen used in higher temperature applications.

The stiffening resins of the present invention are polyimide (PI) basedresins. Polyimides are a generic class of condensation polymers derivedfrom bifunctional carboxylic acid anhydrides and primary diamines. Theycontain an ‘imide’ linkage, —CO—NR—CO—, as a linear or heterocyclic unitalong the polymer backbone. The heterocyclic structure is usuallypresent as a five or six membered ring condensed with benzene(phthalimides) or naphthalene (naphthalimides).

Polyimides include but are not limited to the generic classes ofpolyamideimides (PAI), polyetherimides(PEI) and polybismaleimides(PBMI). PAl's are high performance polymers containing both the ‘imide’linkage and ‘aminde’ (—CO—NH—) linkages in the polymer backbone. PAIresins are produced by solution condensation of aromatic trichlorideswith aromatic diamines. PEI's are condensation polymers produced fromreactions of bisphenols and dinitrobisimides. PEI polymers contain boththe ‘imide’ and ‘ether’ (—O—) linkages in the polymer backbone. Theether linkage improves ease of processing and flexibility. PBMI's aregenerally prepared from the two-step reaction of maleic anhydride anddiamines in the presence of acetic anhydride and catalytic amounts ofnickel acetates and triethylamine. Polyimides include both thermoset andthermoplastic polymers that can be ‘engineered’ to suit specific end-useperformance requirements.

Compared with conventional resins they are significantly more expensive,but do offer the required increased thermal resistance propertiesnecessary for withstanding temperatures ranging from 500° F. up to 700°F. Further, the glass transition temperatures (Tg's) of theseresins/polymers are greater than the maximum operating temperature in anindustrial baghouse, which is typically 500° F. These resins are knownto retain good structural integrity while operating at highertemperatures for extended periods of exposure while maintaining therequired chemical resistance and tensile strength needed. In addition,these resins have excellent resistance to stretch or creep at elevatedtemperatures and are extremely flame retardant. Polyimides are availableas both organic or water based solutions.

A class of polyimide, PAI is discussed for purposes of illustration butis in no means meant to be limiting. A preferred PAI resin is formulatedby the following reaction pathway:

The preferred PAI shown above is a thermoplastic polymer with a Tg of540° F. It is flame resistant, with an LOI (limiting oxygen index) of45%, has low smoke generation and is primarily a water-base system.Although there is a small quantity of solvent found in this PAIformulation, it does not need special processing equipment. Traditionalnonwoven wet chemical processing equipment to control ventilation andexhaust is adequate, and special solvent recovery systems are notnecessary. This helps to hold down the cost of using these types ofresins, allowing these resins to be processed by existing media vendorsand afforded by the dust pollution control industry.

The polyimide stiffening agent of the present invention may be utilizedin combination with any substrate capable of withstanding the conditionsof the particular filtering application. In high temperatureapplications, acceptable polymer substrates include but are not limitedto polyarylene sulfides, polyimides, aramids, polyamides, glass andmixtures or blends thereof. A substrate that is particularly well suitedfor high temperature applications is a blend of polyphenylene sulfide(PPS), which is a subclass polyarylene sulfides, with greater than 10%aramid, polyimide, acrylic, pre-oxidized acrylic or similar polymer, andmixtures thereof.

Another suitable substrate is an aramid fiber medium. Commerciallyavailable aramid based products are most often made from 100% aramidfiber needlefelts that are resin stiffened with conventional resins asis disclosed above. Aramid polymers and blends thereof are generallyselected when a high temperature gas stream needs to be cleaned withtemperatures in the range of 300° F. to 400° F.

A 2% to 20% weight pick up of the polyimide resin is preferred, with 6%to 10% more preferred. Most preferred is a 6% to 7% weight pick up ofthe preferred PAI on the polymer needlefelt media, which is sufficientto allow pleating and pleat retention. This improves over conventionalstiffening agents, available as emulsions and dispersions, that require10% to 25% weight pick up to achieve the same stiffness. The reductionin the weight pick up required by the polyimide resins provides asuperior stiffening agent and renders the polyimide resins moreeconomical.

The polyimide resins provide sufficient stiffness at lower weight pickups primarily due the following: First, the polyimide resin is insolution, rather than an emulsion or dispersion. This allows a thinner,more uniform impregnation of the substrate with the polyimide resin.

Second, the polymer needlefelt is calendered under heat and pressure toattain the thickness and air permeability properties needed from thefinal filter media. In a preferred embodiment the media is calenderedafter singeing but prior to application of the stiffening resin toprevent cracking of the stiffening resin. Calendering the substrateprior to application of the stiffening agent contributes to theuniformity of stiffening resin at lower weights than are required withtraditional stiffening resins.

The polyimide resin may be applied to the substrate by immersioncoating. The substrate is fed into a coating tank and dip coated in theresin solution to impregnate both sides of the substrate. The mediapasses between two nip rolls to remove the excess and enters a dryingoven where the polyimide resin is substantially cured.

A further advantage of the polyimide resin is the fact that it isavailable in a solution, rather than an emulsion or dispersion such asthe prior art resin. In the prior art, the emulsion or dispersion thatis removed by the nip rolls during the immersion coating process isdiluted, as the resin is trapped in the substrate. Since the polyimideresin is in solution, the solution maintains a constant concentration.

In the preferred embodiment the treated substrate is then pleated by anysuitable method, including a blade or push bar pleater. Due to thehigher Tg of polyimides, the temperature during pleated is raised toapproximately 430° F. dependent on the particular polyimide used, as iswell known in the art.

In an alternate embodiment, these types of media can be laminated withexpanded microporous PTFE, as is known in the art, to impart an improvedlevel of filtration performance to the woven or non-woven filter medium.This requires a level of further processing to allow the media to bondto the PTFE membrane. In another alternate embodiment the polymersubstrate may be formed around a support scrim, such as PTFE. Both ofthese processes are well known in the art.

EXAMPLE 1

A 10.0 oz unsupported aramid (Nomex) media, supplied by Southern FeltCo. of North Augusta, S.C. was produced by known methods. The fiberswere processed into a felt and both sides were singed and calendered byconventional methods prior to resin application. The 2376 PAI resinsolution was supplied by Whitford Inc., West Chester, Pa. at 11.0%solids, 10% NMP and 79% water and additional trace components includingsurfactants. The 2376 resin solution was diluted to 5.5% solids usingwater at a 1:1 ratio. The felt was immersed in the resin and the excessremoved by nip rollers to produce a 6% to 7% pickup after the firstpass. The treated media was dried at 425° F. and pleated by conventionalmeans, although the temperature was increased to about 430° F. on theexit platens due to the high Tg of PAI.

A SEM photo of the aramid substrate treated with the preferred PAI isshown in FIG. 5. It is apparent that the PAI resin has coated the fibersindividually and consistently, as the fiber structure is apparent. Forpurposes of comparison, an SEM photo of an aramid substrate treated witha conventional epoxy would be expected to illustrate that the epoxy hadcollected more heavily in the voids between the fibers. Theconcentrations of epoxy not associated with a fiber would be easily‘blown’ off the filter medium resulting in an immediate drop instiffness and pleat stability.

The PAI treated aramid filter was tested in a full scale test baghouseunder industrial conditions. The test baghouse is fully automated andsimulates industrial conditions at a constant temperature, 375° F., 1200cfm (4:1 a/c), 60 psi with dust loading. The pressure drop was constant,ranging between 2.8 and 3.6 inches. The filters were removed at 30,000,50,000 and 110,000 pulses. The results of these tests are shown in theTable 1. Most notably, the data shows that after 110,000 pulses themedia with the preferred resin still had 90% of its original strength.The simulated baghouse testing indicates at 110,000 pulses there is morethan 12 months simulated life on these filters. Further, the test dataindicates the PAI resin is more stable and suitable at withstanding theforces and temperatures expected of the media and can be extrapolated tosuggest it will also be in good condition after 200,000 pulses or theequivalent of two years life while operating on a twenty-four hour/sevenday a week basis.

TABLE 1 ThermoPleat Analysis - BEST Tested Media TM Zero 30,000 50,000Property Units Pulses Pulses Pulses Basis Weight - oz/sq.yd n/a 12.414.9 Dirty Top Basis Weight - oz/sq.yd n/a 11.6 12.5 Dirty Bottom BasisWeight - oz/sq.yd 10.5 11.0 11.4 Clean Top Basis Weight - oz/sq.yd 10.510.8 11.6 Clean Bottom Thickness - Clean inches 0.046 0.049 0.055 AirPerm. Dirty cfm n/a 4.99 7.1 Top Air Perm. Dirty cfm a/a 8.60 12.1Bottom Air Perm. Clean cfm 29.0 28.5 25.7 Top Air Perm. Clean cfm n/a26.5 20.0 Bottom MD Tensile lbs/2″ 188.5 199.5 181.3 Strength XMDTensile lbs/2″ 311.5 314.7 294.0 Strength MD Elong'n at % 57.8 37.2 40.4break XMD Elong'n at % 51.0 42.5 35.1 break MD Tear Strength lbs 23.011.25 11.91 XMD Tear lbs 11.0 7.9 7.5 Strength Retained MD Tensile % 100100 96.2 Retained XMD % 100 100 94.4 Tensile Prior Resin Tensile % 10050 20 Retained MD Tear % 100 49.0 51.8 Retained XMD % 100 71.8 68.2 TearPrior Resin Tear % 100 35 10

The combined average tensile strength and the combined average tearstrength of the PAI resin treated aramid substrate as compared to anaramid substrate treated with a conventional epoxy resin is shown inFIGS. 7 and 8, respectively. As is noted, the epoxy resin treated aramidsubstrate experienced a total filter failure prior to 110,000 pulses.The improved resin treated aramid was flame tested and compared toconventional stiffening agents. Flammability Testing in accordance withFTMS 191A-Method 5903 is shown in Table 2. As is seen, the PAI treatedaramid exhibits excellent flame retardation.

TABLE 2 Flammability Testing Testing in accordance with FTMS 191A -Method 5903 PAI Coated Aramid PAI Coated PPS/Polyimide After Flame(secs) MD 0.0 2.6 XMD 0.0 3.4 After Glow (secs) MD 0.0 0.0 XMD 0.0 0.0Char Length (in) MD 1.8 2.9 XMD 2.0 2.9

An 11.0 oz unsupported filter medium of 75% PPS and 25% polyimide,supplied by Southern Felt Co. of North Augusta, S.C., was produced byknown methods. The fibers were processed into a felt and both sides weresinged and calendered by conventional methods prior to resinapplication. The 2376 PAI resin was supplied by Whitford Inc., WestChester, Pa. at 11.0% solids, 10% NMP and 79% water. The 2376 resinsolution was diluted to 5.5% solids using water at a 1:1 ratio. The feltwas immersed in the resin and the excess removed by nip rollers toproduce a 6% to 7% pickup after the first pass. The treated media wasdried at 400° F. and pleated by conventional means, although thetemperature was increased to about 430° F. on the exit platens due tothe high Tg of PAI.

An SEM photo of the PPS/polyimide substrate treated with the preferredPAI is shown in FIG. 6. It is apparent that the PAI resin has coated thefibers individually and consistently, as the fiber structure isapparent.

The improved resin treated aramid was acid cycling tested, the resultsare shown in the graph of FIG. 7. Flammability Testing in accordancewith FTMS 191A-Method 5903 is shown in Table 2.

All flammability testing was accomplished by Textile Technology Centrein St. Hyacinthe, Quebec.

The presently preferred embodiments have been described, however, manyvariations are possible. As noted above, the polyimide treated polymersubstrate filter of the present invention may be advantageously usedwith the illustrated baghouse, however, the present invention may beused with any suitable baghouse or filtering apparatus. Moreover,although the stiffening resin of the present invention is particularlyadvantageous when used with a pleated filter medium, the invention isequally applicable to filter media of varying structures andapplications, such as woven glass.

The stiffening resin of the present invention has been shown incombination with a non-woven polymer filtering substrate. However, anysubstrate, woven or non-woven, that satisfies the filtering requirementsof the particular filtering application may be used.

While the filtering medium of the present invention overcomes thelimitations of high temperature filtering applications, the presentinvention may also be used in lower temperature applications.

Having described the invention in detail, those skilled in the art willappreciate that modifications may be made of the invention withoutdeparting from its spirit and scope. Therefore, it is not intended thatthe scope of the invention be limited to the specific embodimentsdescribed. Rather, it is intended that the scope of the invention bedetermined by the appended claims and their equivlents.

1. A method of making a filter medium for use in a filtering applicationat an application temperature comprising: providing a substrate;providing a polyimide stiffening agent in solution selected from thegroup consisting of polyetherimides and polybismaleimides; diluting thepolyimide stiffening agent solution to approximately 5.5% solids;treating the substrate with the polyimide stiffening agent solution;curing the treated substrate; and pleating the treated substrate, aftercuring the treated substrate, at a temperature above the applicationtemperature, wherein the application temperature is greater than about375° F.; wherein the treated substrate with the polyimide stiffeningagent is capable of withstanding at least 100,000 cleaning pulses at atemperature of about 375° F., a flowrate of about 1200 cubic feet perminute, and a pressure of about 60 psi.
 2. The method of claim 1,further including calendering the substrate after the step of providinga substrate and before the step of treating the substrate.
 3. The methodof claim 1, wherein the substrate is selected from the group consistingof polyarylene sulfides, aramides, polyimides, glass, acrylics,pre-oxidized acrylics and mixtures thereof.
 4. The method of claim 1,wherein the polyimide stiffening agent is about 2% to about 20% byweight of the total weight of the filter medium.
 5. A method of making afilter medium for use in a filtering application at an applicationtemperature comprising: providing a polymer substrate; calendering thepolymer substrate; providing a polyimide stiffening agent in solutionselected from the group consisting of polyetherimides andpolybismaleimides; diluting the polyimide stiffening agent solution toapproximately 5.5% solids; treating the calendered polymer substratewith the polyimide stiffening agent solution; curing the treated polymersubstrate; and pleating the treated substrate, after curing the treatedsubstrate, at a temperature of about 430° F., wherein the treatedpolymer substrate with the polyimide stiffening agent is capable ofwithstanding at least 100,000 cleaning pulses at a temperature of about375° F., a flowrate of about 1200 cubic feet per minute, and a pressureof about 60 psi.
 6. The method of claim 5, wherein the polymer substrateis selected from the group consisting of polyarylene sulfides, aramides,polyimides, acrylics, preoxidized acrylics and mixtures thereof.
 7. Themethod of claim 5, wherein the polyimide stiffening agent is about 2% toabout 20% by weight of the total weight of the filter medium.
 8. Themethod of claim 5, wherein the application temperature is greater thanabout 375° F.